Halides, chalcogenides and heavy-metal oxides are considered as the most promising materials for manufacturing extremely low-loss glasses for the transmission of light radiations at wavelengths ranging from 2 to 12 .mu.m.
More particularly, attenuation values of the order of 10.sup.-4 dB/km are theoretically attainable so that optical transmission systems with trunk lengths of several thousand kms without intermediate repeaters may be implemented.
A method of fabricating halide-based optical-fiber preforms and, more particularly, fluoride-based preforms, has been described in a papers entitled "Infrared Optical Fibers" by Tadashi Myashita and alia", IEEE Journal of Ouantum Electronics, Vol. QE-18, No. 10, October 1982.
According to this method, named "build-in casting", the material is heated and, once it reaches a low degree of viscosity, is poured into a cylindrical vessel. When the desired-thickness layer for the cladding fabrication solidifies in contact with the cold wall of the vessel, the still liquid axial portion is poured off and replaced by a material suited to core fabrication. After solidification, the vessel is removed and the resultant preform is then drawn.
An alternative process, intended to eliminate the conicity of the preform obtained according to this earlier technique, is described in "Fluoride Glass Preforms Prepared by a Rotational Casting Process" by D. C. Tran et al, Electronics Letters, 22 July 1982, Vol. 18, No. 15. The vessel into which the fluoride based material is poured for the fabrication of the cladding, is rotated around its axis, so as to obtain a constant-thickness layer by centrifugal force. Then the process is the same as that previously described.
However, these methods have a number of disadvantages, making it impossible to reach the above-mentioned theoretical attenuation limits. Some losses arise which are dependent on local discontinuities at the surfaces of contact between core and cladding, formed at successive time periods. Between these surfaces, impurities of various natures, such as powders and gases, are trapped and localized crystallization can result giving rise to scattering attenuations.
The preform also cannot be very large, since the enclosed region at controlled temperature, necessary to limit the pollution must be able to contain the vessel-rotating apparatus, furnaces, handling apparatus, glass-fabricating apparatus, etc.
As a consequence of the limitation in preform size, the fiber trunks produced cannot be very long in spite of the fact that the low attenuation attainable by these methods could be expected to allow the fabrication of fiber with lengths of several hundreds of kms.
Repeated raw material handlings introduce further unavoidable contaminations even when they involve the most sophisticated methods.
Finally, graded reflective-index fibers are difficult to obtain.